In parabolic reflector antennas, the value of the reflector's diameter is determined by the central operating frequency of the antenna. The lower the antenna's operating frequency, the greater the reflector's diameter, assuming equivalent antenna gain. For deep reflector antennas, the F/D ratio is less than or equal to 0.25. In this report, F is the focal distance of the reflector (the distance between the reflector's apex and its focus) and D is the reflector's diameter. These antennas exhibit high spillover losses and decrease the antenna's front-to-back ratio. Spillover losses lead to environmental pollution through RF waves and must be limited to levels defined by standards.
One common solution is to attach to the periphery of the parabolic reflector a cylindrical wall, also known as a shroud, with a diameter neighboring that of the reflector and which is of a suitable height, most commonly covered with a material that absorbs RF radiation. The use of an expensive absorbent shroud is necessary in order to limit the spillover effect and improve the antenna's performance. Nonetheless, the solution increases the cost and dimensions of the antenna, and makes packaging for transport more complicated.
Furthermore, the presence of the shroud increases the antenna's wind-catching surface and the risk of accumulation of polluting agents. For this reason, the shroud is associated with a radome which exhibits an impermeable protective surface closing off the space defined by the reflector and the shroud from the outside. This radome can be flexible or rigid, flat or not, and in any shape whatsoever. A circular rigid radome, the most commonly used kind today, offers the advantage of good resistance to the outside climate conditions, such as rain, wind, or snow.